1. Field of the Invention
This invention is generally directed to flexible joints for subsea risers and pipelines, and more particularly to a joint providing bending and rotation with respect to a longitudinal axis of the riser or pipeline.
2. Description of the Related Art
Offshore drilling and production facilities use risers or pipelines constructed of rigid pipe sections and flexible joints. This construction, for example, permits a pipeline to be laid along the under-water level and then raised for connection along the length of a supporting leg of an offshore platform. Typically, an uppermost joint near the platform is a swivel joint permitting axial rotation, and the lower joints are flex joints permitting axial bending.
One kind of flexible joint providing both swiveling and bending is a ball joint as described in Oliver U.S. Pat. No. 3,727,954. A disadvantage of such a ball joint is that the joint has no means for resisting bending of the articulated riser. Therefore, the articulated riser may assume a configuration where the joints reach their limits of displacement and permit the buildup of a high degree of stress on the joints and the connection of the riser to the floating platform.
There has been a continuing trend to use offshore drilling and production facilities in increasingly deeper water and in geographical regions such as the North Sea having harsh weather conditions. Offshore drilling and production facilities in such dynamic ocean environments can experience extreme load conditions on riser and mooring system components. Extreme weather conditions alone or in combination with equipment failures may result in rotational motions of the floating platform in multiple degrees of freedom simultaneously.
Floating drilling vessels and production facilities have recently become important components for exploration as well as large scale development of offshore oil deposits. In the case of floating production systems (FPS and FPSO), the vessel in question may be either a semi-submersible platform or a ship. If it is a ship, it will often be attached at one end to a turret which is moored to the seabed, and the vessel is free to rotate about this turret in response to wave, wind, and tidal forces. See, for example, A. Smedal and K. Syvertsen, "STL, A New Approach To Field Development," Advanced Production and Loading, APL AS, 27th Annual Offshore Technology Conference in Houston, Tex., U.S.A., 1-4 May 1995, pages 399-405, incorporated herein by reference.
Drilling ships and exploration vessels are kept on station by dynamic thrusters which are used to counteract the forces on the ship and keep it on station over the drill site. At times, natural forces and mechanical failures have caused such vessels to move off station resulting in severe damage to the riser.
For such floating drilling vessels and production facilities, it is therefore prudent to use an uppermost swivel joint in the riser near the floating vessel, and to use elastomeric flex joints along the riser in order to absorb the rotational and bending motions caused by rotation and displacement of the floating vessel about its mean position. In such instances, an uppermost joint capable of simultaneously accommodating both rotation and bending motions about the riser would be invaluable.
In the case of catenary and other types of risers suspended from tension leg platforms (TLP) and FPS Platforms (which are moored in all quadrants of the seabed), large dynamic motions in yaw, roll and pitch are still possible during extreme weather conditions. In such instances, an uppermost joint located at the riser/platform attachment point and simultaneously accommodating both angular and rotational motions about the riser would enhance the fatigue life of the riser and prevent damage. For these kinds of installations, it would also be helpful if such a joint could have a built-in mean angle identical to the hang-off angle of the riser. This would facilitate self-adjustment during pull-in and coupling of the upper section of the riser to a leg of the TLP or to a TLP or FPS receptacle.